The field of the invention is scanning densitometers, measuring instruments which determine the density distribution of a graphic image. More particularly the invention relates to cooling means and correction circuits which enhance the accuracy of density measurements by minimizing the detrimental effect of thermal noise generated in a diode array which is used to make the density measurements.
Density measurements, the determination of highlight, midtone and shadow denisties on a graphic image, have in the past been a subjective evaluation and determination which is very time consuming. The operator generally used a hand-held densitometer and subjectively chose the brightest, the darkest and other areas of a graphic image for measurement and for entry into a reproduction system. There has been a clear and long felt need for automation of this process.
Density measurements, however, are logarithmic. Consequently on a ten volt measurement scale the following relationships may exist: 10 volts equal 0 density; 1 volt equals 1.0 density; 0.1 volt equals a 2.0 density; 0.01 volt equals a 3.0 density; and 0.001 volts equals a 4.0 density. Therefore, distinguishing densities between 3.5 and 3.6 with available automated technology requires extreme accuracy in measurement and is extremely difficult to achieve.
The measurement is complicated when using charge coupled diode arrays by a phenomena referred to as "dark signal". When the cells of a photodiode array are exposed to light they charge in proportion to the amount of light falling on the cell. During the exposure time the charge is accumulated to a specific charge which is then transferred by gates to a shift register, out of which the data is read for conversion and processing. The accuracy of the measurement is effected by electron noise or thermal noise generated in the cell sites and in the shift register. In a commercially available photodiode array with a 30 millesecond integration time and at 20.degree. centigrade, dark signal and dark signal non-uniformity are proportional to the integration time and approximately doubles for every 7.degree. centigrade increase in temperature. This presents a complimentary problem because not only do the cells accumulate dark signal or thermal noise, they also accumulate it non-uniformly. To obtain accurate measurements, therefore, it is necessary to control the amount of dark signal generated and to compensate to the extent possible for any dark signal or thermal noise which is generated.